This invention relates generally to injection molding and more particularly to thermal gated injection molding apparatus having a cavity forming insert with a combination of heating and cooling extending around a central bore leading to a gate.
For clarification, reference herein to a "cavity forming insert" shall include both a cavity insert as seen in the first embodiment and a core insert as seen in the second embodiment.
Thermal gating which is also referred to as temperature assisted gating involves changing the temperature of the melt in the gate area during each cycle to assist in controlling flow to the cavity. This is known in the art such as in Schmidt U.S. Pat. No. 4,768,945 which issued Sept. 6, 1988 which describes the heating element having a forward portion extending diagonally into a nose portion of the nozzle. In the applicant's U.S. Pat. No. 4,911,636 which issued Mar. 27, 1990 and U.S. Pat. No. 4,941,249 which issued July 17, 1990, thermal gating is described using a nozzle with an integral heating element having a circular portion which encircles the melt bore in a forward nose portion of the nozzle. The applicant's U.S. Pat. No. 4,875,848 which issued Oct. 24, 1989 discloses a tapered gate insert which is mounted in the forward end of a nozzle and is heated by an integral helical heating element.
It is known in the art to use a separate cavity insert rather than a cavity plate in which the nozzle is seated to form the rearward side of the cavity. For instance, the applicant's U.S. Pat. No. 4,911,636 which issued Mar. 27, 1990 show a heated nozzle extending through a support or nozzle plate into a cooled cavity insert. However, the gate and the entire melt passage leading to it are part of the nozzle and all of the heating is provided by the single heating element in the nozzle. Thus, the nozzle must project through the cavity insert to the cavity.
These previous systems have the disadvantage for thermal gating temperature sensitive materials that thermal response is delayed by the cooling being in the cavity insert and the heating in the nozzle.